They are what they are, and as long as they are converted meaningfully into the output space, it doesn't make too much difference how they're used as a working space.

But that is precisiely the point being made - that the "Melissa" preview display, histograms, and color values reflect sRGB (tone-level) values only - and do not reflect ProPhoto RGB or Adobe RGB.

I would assume that the ProPhoto RGB output space is legitimately ProPhoto RGB.

In terms of color-gamut, yes. In terms of preview appearance, histograms, and color values, no.

But, isn't this only as a working space? If so, that's not great, but it's not the same as saying that the conversion on "export" is not to a proper ProPhoto RGB output space. Is that what you're saying?

What I am saying is that the Adobe CR and LR preview displays, histograms, and color value readouts (which are provided using "Melissa RGB") do not (within the linearized low-level region of sRGB and Melissa RGB) accurately correspond (in terms of RGB tone-levels) to what will actually exist if/when the in-process image is then rendered into ProPhoto RGB or Adobe RGB 1998 (but not sRGB).

The failure to accurately preview and report low-level signal and image-noise seems rather undesirable as a characteristic - because it provides an overly optimistic preview of existing image-noise levels.

As to using sRGB as a work-around, I wouldn't want to limit myself to that in many circumstances.